Process paper on lost foam casting iron cap for high voltage wire


1: the use of iron cap

Iron cap is the circuit transmission, lightning arrester and substation accessories. The role of insulated and fixed (suspended) conductors in high-voltage overhead transmission and distribution lines. as shown in Figure 1.

  

 

Fig. 1 Disc Type Suspension Insulator for High Voltage Line

2 Coated sand casting production mode of the original process

 

2.1 the characteristics of the original process

  

 

Fig. 2 Casting Process of Coated Sand with 120 Model Iron Cap

Figure 3 Q70 iron cap coated sand casting process

Coated sand casting method as shown in Fig. 2, Fig.3As shown in the figure, two or four pieces are poured at one gate. The pouring position of the casting is to place the large mouth of the casting horizontally. The position of the inner gate is to introduce molten iron from the back head of the casting and have a riser at this position for slag collection and feeding. Pouring temperature below1350degree will appear cold insulation phenomenon. The process yield in the production mode of coated sand is in.40%Around, the yield is in.90%Above.

Advantages: the process is more mature and stable, producing qualified castings. The use of coated sand, do not have to brush paint casting surface finish and more suitable for the production of such small castings.

Disadvantages: the use of disposable coated sand casting production, high cost. The environment of the molding core clamping process is poor, and only two pieces are poured at a time, and the subsequent grinding and cleaning are also relatively labor-consuming and time-consuming, and the process yield is low.

Melting and pouring of 2.2 original process

(1) Ingredients

Pg iron: scrap steel: return charge = 40%:10%50%

(2) Sphering and inoculation

The spheronizing agent is 8-3 model spheronizing agent produced by Pucheng Perseverance Metal Casting Material Co., Ltd., and the amount of spheronizing agent added is the composition of molten iron.1.3%.

The inoculant adopts ordinary 75SiFe, and the addition amount is1.1%.

Spubling inoculation method: bag bottom inoculation.

 

(3) Parameters of tapping and pouring

 

Table 1 Parameters of tapping and pouring

  (4)QT450-10Composition and Organization Requirements

 

Table 2 QT450-10 composition

Tissue requirements: as-cast pearlite content of about 20%. Carburizing bodies are not allowed, otherwise heat treatment is required to eliminate carburizing bodies.

2.3 product quality requirements

The casting is free of casting defects such as cold insulation, slag inclusion and sand inclusion. The strength and elongation of the test bar meet the requirements of the QT450-10 grade. Carburizing body is not allowed in the organization. The tensile test of the casting is 100% qualified.

3 Trial production of iron cap by EPC

3.1 Brief introduction of two kinds of castings in this test

In this process experiment, 120 models andQ70Two models of iron caps. Because the market demand of these two models is the largest, and the wall thickness is the thinnest in all the iron cap castings of the customer, so choose these two models to do the experiment.

120 model iron cap: materialQT450-10, single weight0.84kg, minimum wall thickness3mm, casting tensile test requirements in120KNAbove, the overall structure of the structure of the casting is more compact, the wall thickness of the fat belly part is3mmThe lips are thicker.

Q70 model iron cap: materialQT450-10, single weight0.95kg, the minimum wall thickness is4mmThe tensile test of castings requires the tensile force to be85KNAbove. Due to the fierce competition and the improvement of customer requirements, the surface of the casting is not allowed to have obvious pits and protrusions, and the large round mouth is required to be rounded. In terms of structure, the overall structure is not so compact compared with the 120 type of iron cap, and the wall thickness of the fat belly part is4mmThe wall thickness of the mouth and lips is thin.

3.2 Difficulties in Production by Lost Foam Casting Process

1. The casting wall thickness is thin only3-4mmLeft and right, it is easy to produce cold insulation.

 

2. The temperature is not easy to control, the thinner wall thickness requires the pouring temperature to be increased, but it will affect the spheroidization effect, and may produce shrinkage defects in the thick wall part of the ball head.

3. Adopt the lost vanishing mold string casting process scheme. The formulation of the scheme shall consider ensuring sufficient filling speed for each casting on the string during filling, and prevent the liquid metal from breaking in the process of filling the casting. If the corresponding filling speed is not guaranteed, the casting will definitely have cold insulation, resulting in waste products.

4. The lost foam series casting process is adopted, and many white molds are hung on a string. After the white molds are bonded, they are easy to fall off and crack in the process of drying, coating and burying the box, and casting defects such as slag inclusion, sand sticking and sand inclusion are easy to occur. In addition, if there is no better process plan or fixed tooling for the string casting process, it is possible to use the box group type, which will reduce the production efficiency.

5. The roundness of the opening lip of the white mold is strict and easy to deform.

6. Manufacturer's requirements. Previously has been in the use of coated sand casting production of castings, to ensure that the casting made on the basis of the process yield reached.40%above, the yield is in90%Above. In addition, the energy saving, time-consuming and labor-consuming aspects of the production of such castings by means of lost-energy mode should be greatly improved compared with the previous ones.

4 Lost Foam Process Test Process

4.1 white model

 

The 120 model iron cap white mold model is a combination of upper and lower white molds as shown in the figure.4This requires attention to the possibility of casting scrap due to poor bonding at the parting surface. AndQ70No parting surface for model white mold forming as shown in Fig.5shown.

  

 

Fig. 4 white mold of 120 model iron cap

  

 

Figure 5 Q70 Iron Cap White Mold

4.2 coating and brushing

The use of the company's production of RO-QT coatings. Brush twice, coating thickness in1mmAbout, the drying time for both times is12hAbove.

4.3 casting process aspects and melting and pouring

The main idea: first explore the best casting position and the best in-gate opening position, and then combined with the characteristics of the string casting process in the sand box arrangement of the runner, runner arrangement, so as to adapt to the lost vanish mold process and meet customer requirements of the program.

4.3.1 Casting process scheme

Absorb the experience of the original casting process and then combine it with the lost foam casting process. For this production test, we have developed a total of 5 pouring position program.

From the point of view of finding the best pouring position and the opening position of the in-gate, first of all, we adopt the method of vertical pouring of model clusters, which is a common way for many small castings to be poured in series. The reason is that each layer of castings can be filled from bottom to top in order to ensure the pouring speed of each layer of castings and avoid turbulence.

 

There are 4 layers of model clusters, and castings are hung on each model cluster. The specific scheme is shown in the figure.6shown. Co-pouring5String model clusters, each layer of castings in each string model cluster is placed in the same way, except that the upper and lower castings are in different order to avoid errors caused by different upper and lower positions of castings.

  

 

Fig. 6 Casting Pouring Position

4.3.2 Melting and pouring scheme

(1) The ingredients are unchanged, pig iron: scrap steel: return material.=40%10%50%;

(2) The addition of spheroidizing agent was increased from 1.3 to 1.4;

(3) The addition of inoculant was increased from 1.1 per cent to 1.2 per cent;

(4) The way of spheroidizing inoculation adopts the method of spheroidizing inoculation with the bottom of the bag;

(5) The parameters of tapping and pouring are as shown in the table3as shown;

 

Table 3 Temperature for tapping and pouring of EPC

4.3.3 Pouring results

 

(1) The model clusters are poured in sequence, and there is no problem in the control of the pouring process, and there are no pouring problems such as slag inclusion and sand inclusion in the castings. Through pouring experiment5#The castings obtained from the scheme pouring have no defects, and the other three schemes all have cold isolation defects, and the defect forms are shown in the figure.7shown.

  

Fig. 7 Casting Cold-off Defect

(2) Analysis of the advantages and disadvantages of various pouring schemes

1# pouring scheme

In the cast 5-string model cluster4String scrap. Two inner gates are used to introduce molten iron. The inner gate is located on the front of the ball head, and the casting pouring position is inclined upward.

Analysis reason: the metal liquid is introduced with two internal gates to form two liquid flows. when the metal liquid is fused at the final filling part, the temperature of the metal liquid cannot reach the fusion requirement, resulting in poor fusion and cold isolation.

2# pouring scheme

All the five series of model clusters poured were scrapped, and two internal gates were used to introduce molten iron from the back of the ball head, and the casting pouring position was obliquely upward. The cold isolation position is mainly distributed on the upper part of the thin-walled fat belly at the pouring position, which is exactly the position of the final filling and fusion of the molten metal. The form and cause of the defect are similar to those of the 1# pouring scheme.

3# pouring scheme

The large mouth of the casting is placed horizontally, and the molten iron enters from the back of the ball head. Cold insulation defects appear at the final filling part of the casting and at the uppermost part of the pouring position. When the metal liquid is finally filled and merged, the temperature has not reached the fusion requirement.

4# pouring scheme

Cold isolation defects were found in 4 of the five model clusters cast. A single gate is used to introduce molten iron from the fat belly of the iron cap, with the casting pouring position facing up.

The defect is manifested as the problem of cold insulation and meat when smashing the gate. The position of cold insulation is opposite to the gate and is also the final filling part.

5# pouring scheme

All of the five model clusters were successfully poured. A single internal gate is used for bottom casting, and the metal liquid is introduced from the back of the ball head, and the pouring position is that the large mouth of the casting faces upward. As a result, there were no defects and the finished product was successfully poured.

Compare with other schemes and analyze the reasons: adopting a single internal gate can make the heat more concentrated than multiple internal gates, and there will be no metal liquid convergence in the filling process in the form of bottom casting with large mouth upward. Therefore, in contrast, this large mouth up single inner gate bottom casting process scheme is more suitable for this structure of the casting process requirements. And in the later experiment, even if the temperature is low, only 1370 ℃, the castings poured out have no defects, which shows the stability of this process scheme. The pouring temperature can be poured out from 1350-1450 ℃. From this point of view, it also leaves a lot of freedom for the actual production in terms of pouring temperature.

 

Summarizing the results of the above five series of pouring, the best pouring position and gate position of the casting should be 5# scheme. In addition, practice has proved120The type of iron cap because of its compact structure, large mouth lip part of the thick, thin-walled area is small, so no matter what kind of casting process can be cast successfully. Therefore, the casting process is closely related to the casting structure. Finished casting as shown8shown.

  

Fig. 8 Casting Finished Product

The composition, metallographic and tensile tests shall be conducted for the 4 finished products without appearance defects. The obtained composition, microstructure and stress all meet the requirements.

5 Series Cast Process

The next step is the connection scheme between the casting sprue and the runner so as to realize the string pouring and improve the production efficiency. The key to the string casting process is how to guide the molten iron to be introduced into the casting cavity in sequence, so that the necessary filling speed can be met in the filling process of each casting, and turbulence does not occur when the molten metal opens each inner gate in sequence, thus avoiding cold isolation defects. So as to find the best flow mode when the metal liquid is introduced into the cavity, and find the best process yield and yield point.

 

In this regard, we have developed a total of five options to choose from, as shown in Figure 9-13 below.

  

Fig. 9 String-casting scheme1

  

Fig. 10 Scheme of Series Cast2

  

Fig. 11 Scheme of Series Cast3

Fig. 12 String-casting scheme4

    

 

Fig. 13 String-casting scheme5

According to the characteristics of casting process, we analyze the advantages and disadvantages of the above process. 1# and2#The requirements of sequential filling cannot be met. When the casting reaches the end of the branch, the crawling distance of the molten metal is long and the temperature drop is large. In addition, during the filling process, the lower layer is not filled and the upper layer starts to fill, thus affecting the rising speed of the lower layer of the molten metal, resulting in cold isolation defects. In actual experiments, the castings of these two processes also have large cold isolation defects. Therefore, the two processes can not meet the production requirements in terms of process, and the scrap rate is high.

In the 3# scheme, we use the principle of simultaneous filling to design the runner, and the reject rate of castings in the pouring result is higher.90%above, but the process yield is low, approx.45%Left and right, so this kind of scheme is not satisfactory.

In the 4# scheme, we use the principle of sequential filling to design the sprue, and design the pouring process when climbing.4#The process plan is the same, the yield is high, but because the large space in the sand box is not used, and the use of the main runner greatly reduces the yield, the process yield is insufficient.35%.

5# scheme we take a single string of single-cast way, eliminating the process above the runner, so as to improve the process yield of the way, can ensure the yield of finished products, and casting process yield increased65%Above, thus meeting customer needs.

6 Conclusion

 

This paper explores the feasibility of the iron cap casting from the original coated sand casting to the lost foam casting method, from the initial search for the best inner gate position and pouring position to the back of the search for the best string pouring method, combined with the improvement of melting and pouring process, so as to successfully explore the feasibility of the lost foam casting production of iron cap castings.

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